Field of the Invention
The invention relates to a method for operating a semiconductor memory having a plurality of memory areas, in which information items are written to memory addresses and are read from the memory addresses again, and in which an error check on the information items is performed.
The invention also relates to a semiconductor memory having a plurality of memory areas that each have a multiplicity of memory addresses to which information items can be written and from which information items can be read.
In integrated semiconductor memories, a large number of memory cells store information items in the form of quantities of charge. The stored information items have to remain stored over a long time in the case of nonvolatile semiconductor memories, and in the case of volatile semiconductor memories they need to remain stored at least for the period of time up to the next refresh operation. Tolerances for fabricating integrated semiconductor circuits and not completely avoidable leakage currents can result in such charges flowing away. Improved manufacturing technologies can eliminate such influence.
External influences, particularly alpha radiation, affect the reliability of the information storage. Alpha particles penetrating an integrated semiconductor memory produce secondary charges that flow away in an uncontrolled manner and can thus alter quantities of charge stored in memory cells. Since the penetration of alpha radiation cannot be avoided, manufacturers of semiconductor products specify error tolerances that stipulate an upper limit for the likelihood of an error when storing information items. Particularly in safety-related circuits, the probability of failure must be only a few storage errors per million memory cells.
In view of the errors that cannot be ruled out completely when operating an integrated semiconductor memory, complex ECC (Error Code Correction) modules are used in which time-consuming error corrections are made. By way of example, checksums are formed that can be used to identify erroneous data bits. The modules include a plurality of semiconductor chips that are disposed on a printed circuit board, for example. These are used at intervals of time to perform reconditioning operations on the memory information items, “memory scrubbings”, in order to eliminate an accumulation of errors in individual memory cells before errors occur in adjacent or associated memory cells which can no longer be corrected. The technical involvement, the cost and the time involved for such external corrective measures in the semiconductor memory are very high.